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The different types of lipid-linked oligosaccharide (LLO) precursor produced in different organisms.. N-linked glycosylation is the attachment of an oligosaccharide, a carbohydrate consisting of several sugar molecules, sometimes also referred to as glycan, to a nitrogen atom (the amide nitrogen of an asparagine (Asn) residue of a protein), in a process called N-glycosylation, studied in ...
N-linked glycosylation is a very prevalent form of glycosylation and is important for the folding of many eukaryotic glycoproteins and for cell–cell and cell–extracellular matrix attachment. The N-linked glycosylation process occurs in eukaryotes in the lumen of the endoplasmic reticulum and widely in archaea, but very rarely in bacteria.
The most important of these to note are N-linked glycosylation and disulfide bond formation. N-linked glycosylation occurs as soon as the protein sequence passes into the ER through the translocon, where it is glycosylated with a sugar molecule that forms the key ligand for the lectin molecules calreticulin (CRT; soluble in ER lumen) and ...
The most common method of glycosylation of N-linked glycoproteins is through the reaction between a protected glycan and a protected Asparagine. [5] Similarly, an O-linked glycoprotein can be formed through the addition of a glycosyl donor with a protected Serine or Threonine. [5] These two methods are examples of natural linkage. [5]
The process of N-linked glycosylation occurs cotranslationally, or concurrently while the proteins are being translated. Since it is added cotranslationally, it is believed that N -linked glycosylation helps determine the folding of polypeptides due to the hydrophilic nature of sugars.
N-glycosyltransferases are an unusual [a] type of glycosyltransferase which joins single hexoses to the target protein. [6] [7] [4] Attachment of sugars to the nitrogen atom in an amide group — such as the amide group of an asparagine — requires an enzyme, as the electrons of the nitrogen are delocalized in a pi-electron system with the carbon of the amide.
N-linked glycosylation begins in the rough endoplasmic reticulum (ER), where a precursor oligosaccharide is synthesized on a lipid carrier called dolichol. The precursor consists of a core structure made up of two N-acetylglucosamine (GlcNAc) residues, nine mannose (Man) residues, and three glucose (Glc) residues. [7]
Specifically, in a reaction involving uridine diphosphate (UDP) and N-acetylglucosamine, the enzyme N-acetylglucosamine-1-phosphate transferase catalyzes the N-linked glycosylation of asparagine residues with M6P. Once appropriately marked with the M6P targeting signal, these proteins are moved to the trans-Golgi network.